Download El rol de la respiración aeróbica en el ciclo de vida de Escherichia coli

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EN CIENCIAS E INGENIERÍAS
COMUNICACIÓN/COMMUNICATION
SECCIÓN/SECTION B
The role of aerobic respiration in the life cycle of Escherichia coli:
Public health implications
Gabriela Vasco1, Thea Spindel1, Sara Carrera1, Anne Grigg1, Gabriel Trueba1∗
1 Microbiology
∗ Autor
Institute, Universidad San Francisco de Quito, Quito, Ecuador.
principal/Corresponding author, e-mail: [email protected]
Editado por/Edited by: Diego F. Cisneros-Heredia, Ph.D.(c)
Recibido/Received: 2015/06/02. Aceptado/Accepted: 2015/10/05.
Publicado en línea/Published on Web: 2015/12/30. Impreso/Printed: 2015/12/30.
Abstract
The anaerobic intestinal setting of warm-blooded animals is considered E. coli’s main
habi-tat. However E. coli transmission to new hosts requires the release of bacterial cells to
an aerobic environment; we postulate that E. coli uses aerobic respiration to multiply in
fecal matter during this phase of the cycle. To test this idea, we incubated fresh chicken
fecal matter in aerobic and anaerobic settings and showed that E. coli counts increased
signifi-cantly when fecal matter was incubated in the presence of oxygen. Our results
suggest that aerobic growth in fecal matter outside of the host may be a crucial phase of E.
coli’s natural cycle. This feature may extend to pathogenic members of the
Enterobacteriaceae family.
Keywords. Escherichia coli, aerobic respiration, fecal matter, environment.
El rol de la respiración aeróbica en el ciclo de vida de Escherichia coli:
Implicaciones para la salud pública
Resumen
Se piensa que el hábitat principal de E. coli, es anaerobio (los contenidos intestinales de
animales de sangre caliente). Sin embargo la transmisión de E. coli de un hospedador a
otro requiere la eliminación de esta bacteria (por medio de las heces) a un medio rico en
oxígeno. Nosotros proponemos que E. coli usa la respiración aerobia para multiplicarse
masivamente en las heces. Para probar esta hipótesis nosotros incubamos heces fecales de
aves en presencia y ausencia de oxígeno. Nuestros resultados sugieren que el crecimiento
aerobio en las heces es una parte crucial en el ciclo natural de esta bacteria. Esta característica puede ser común a otros miembros de la familia Enterobacteriaseae.
Palabras Clave. Escherichia coli, respiración aeróbica, heces fecales, ambiente.
Introduction
The anaerobic setting within the large intestine of warmblooded animals is considered the primary habitat of Escherichia coli; here E. coli populations readily replicate.
However, a crucial part of E. coli’s life cycle is fecaloral transmission between animal hosts. Large numbers
of E. coli cells are released to the environment (with fecal matter) and they reach the new host through water
or food. Outside the hosts these bacteria are exposed to
a largely aerobic setting.
Escherichia coli transmission to other hosts depends mainly
on environmental conditions and its aptitude to survive
in this aerobic environment [1], although some E. coli
strains may have evolved to grow outside of the host
[2]. At the same time most E. coli strains survive short
periods of time outside the host [1], this is why E. coli
is considered a good indicator of fecal contamination.
Escherichia coli, and other members of the family Enterobacteriaceae, use different metabolic pathways (anaerobic respiration, microaerobic respiration and fermentation) to obtain energy from organic compounds (in animal’s guts), under either low oxygen or anaerobic conditions [4–6]; therefore it is intriguing that aerobic respiration is a universal property in E. coli strains.
Although aerobic respiration is the most efficient way
to obtain energy from organic matter [3, 4], it is energetically demanding as it requires many genes and the
synthesis of several molecules that are part of complex
systems such as: signal transduction, gene transcription
and electron transport [1, 4, 6]. It is paradoxical that a
bacterium which replicates mainly in a microaerophilic
and anaerobic habitats, conserves genes for growth under fully aerobic conditions; bacterial genes are prone
to inactivation unless there is a strong selective pressure
Avances en Ciencias e Ingenierías, 2015, Vol. 7, No. 2, Pags. B1-B3
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Av. Cienc. Ing. (Quito), 2015, Vol. 7, No. 2, Pags. B1-B3
Figure 1: Boxplot using log 10 scale shows counts of Escherichia
coli colonies per gram of fresh fecal matter (First day) and in fecal
matter incubated in the bench for 24 hours at room temperature.
Asterisk indicates statistically significant difference as assessed by
the t-test (t=6.19, p-value=4.469e-05).
[7].
In the other hand, a previous report has demonstrated
that E. coli can grow in fecal matter outside of the host
and the authors suggested that aerobic respiration may
be important component of this growth [8]. In this study
we tested the hypothesis that E. coli uses oxygen to
quickly multiply in fecal matter. We argue that using
its resources for this quick amplification may enhance
its chances of finding another host.
Materials and Methods
Fresh stool samples were obtained from a chicken [n=8].
Each sample was separated in two aliquots and weighed
separately. The first aliquot was immediately plated out
and cultured for 24 hours at 37 ◦ C in Chromocult Coliforms agar (Merckr); E coli was identified by β- glucoronidase activity. The second aliquot was placed into
a Petri dish and left in a 21 ◦ C after which this aliquot
was cultured as previously.
In a subsequent experiment, fresh chicken stool samples
were collected [n=10], separated in two aliquots and
weighed. One part was placed in an open pouch and
the other was placed in an anaerobic pouch (BD GasPak EZ Anaerobe Pouch System, 260683). The samples
were left for 24 hours in a 21◦ C environment. E. coli
were enumerated as in the previous experiment. Statistical analyses and figures were carried out in the RStuc 2009-2014 RStudio software Version 0.98.1091 - dio, Inc.
Results y Discussion
Colony counts in fecal samples incubated for 24 hours at
room temperature were significantly higher than colony
counts of fresh fecal matter (Figure 1). To determine
whether oxygen was involved in the high growth rates
outside the intestine, a second experiment was carried
out under both an aerobic and anaerobic conditions. The
samples exposed to an aerobic environment showed a
significant higher growth than those cultured under anaerobic conditions (Figure 2).
Vasco et al.
Figure 2: Boxplot using log 10 scale shows counts of Escherichia
coli colonies per gram of feces after 24 hours of growth in anaerobic and aerobic conditions. Asterisk indicates statistically significant difference as assessed by the t-test (t=4.01, p-value= 0.00154).
Our experiments support the idea that the E. coli population multiply in fecal matter in the presence of oxygen, which may increase the chances of E. coli to colonize new hosts. The approximate increase in colonies
(at least 10 fold) suggests that aerobic replication in fecal matter is a critical part in E. coli’s natural cycle (and
possibly in the cycles of other members of Enterobacteriaceae including pathogens such as Salmonella).
Escherichia coli is a minor component of the animal
microbiome probably because of high competition with
anaerobic bacteria such as Bacterioides or Clostridium;
however, once the intestinal content is exposed to oxygen, E. coli gains the competitive advantage and overgrows anaerobic bacteria. We propose that aerobic growth
in fecal matter is a critical phase in E. coli’s natural life
cycle. These findings underline the importance of a neglected phenomenon which has large implications in environmental sanitation and public health.
Acknowledgments
The authors thank Joe Eisenberg for his editorial suggestions. This research was funded by Universidad San
Francisco de Quito.
References
[1] Savageau, M. 1983. “Escherichia coli habitats, cell
types, and molecular mechanisms of gene control”.
American Naturalist, 122: 732-744.
[2] Ksoll, W.; Ishii, S.; Sadowsky, M.; Hicks, R. 2007.
“Presence and sources of fecal coliform bacteria in
epilithic periphyton communities of Lake Superior”.
Applied and Environmental Microbiology, 73: 37713778.
[3] Hadjipetrou, L.; Stouthamer, A. 1965. “Energy production during nitrate respiration by Aerobacter aerogenes”. Journal of General Microbiology, 38: 29-34.
[4] Bueno, E.; Mesa, S.; Bedmar, E.; Richardson, D.; Delgado, M. 2012. “Bacterial adaptation of respiration from
oxic to microoxic and anoxic conditions: redox control”. Antioxidants & Redox Signaling, 16: 819-852.
Vasco et al.
[5] Jones, S.; Gibson, T.; Maltby, R.; Chowdhury, F.; Stewart, V.; Cohen, P.; Conway, T. 2011. “Anaerobic respiration of Escherichia coli in the mouse intestine”. Infection and Immunity, 79: 4218-4226.
[6] Govantes, F.; Albrecht, J.; Gunsalus, R. 2000. “Oxygen regulation of the Escherichia coli cytochrome d oxidase (cydAB) operon: roles of multiple promoters and
the Fnr-1 and Fnr-2 binding sites”. Molecular Microbiology, 37: 1456-1469.
[7] Mira, A.; Ochman, H.; Moran, N. 2001. “Deletional
bias and the evolution of bacterial genomes”. Trends in
Genetics, 17: 589-596.
[8] Russell, J.; Jarvis, G. 2001. “Practical mechanisms for
interrupting the oral-fecal lifecycle of Escherichia coli”.
Journal of Molecular Microbiology and Biotechnology,
3: 265-272.
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